1. Field of the Invention
The present invention relates to a pneumatic tire in which belt layers are buried in a tread portion, and more particularly to a pneumatic tire which enables durability thereof to be improved by preventing oxidation degradation of the belt layers.
2. Background Art
Separation between a cord and coating rubber at ends of the belt layers in the axial direction of the tire can be cited as one example of a cause of decrease in durability of a pneumatic tire in which the belt layers are buried in a tread portion. Particularly, the deterioration of the belt layers tends to cause the separation between a cord and coating rubber. It has been pointed out that, because oxygen in the air oxidizes coating rubber of belt layers, such deterioration of belt layers is accelerated in a process, in which air in a tire leaks from the tire.
From this viewpoint, inhibiting of oxidation degradation of coating rubber of belt layers in the following manner has been proposed. A cord reinforcement layer is buried in a buttress portion of a pneumatic tire. The cord reinforcement layer is formed by arranging a plurality of cords in the axial direction of the tire. The cord reinforcement layer consists of a belt inner portion extending in a region below the belt layers, an extending portion extending outward in the axial direction of the tire, and a close portion close to the outer surface of the tire. Thus, air moving in the tire is lead to the outside of the tire through a cord reinforcement layer and thereby a large amount of oxygen can be prevented from passing near the ends of the belt layers. Hence, the oxidation degradation can be inhibited (refer to, for instance, Japanese patent application Kokai publication No. 2003-80905).
However, the following problem is brought about in the case where a cord reinforcement layer, which is formed by arranging a plurality of cords in the axial direction of a tire, is buried in a buttress portion of the tire, and where an end of the cord reinforcement layer is made close to the outer surface of the tire. Specifically, the problem is that moisture is supplied to belt layers through the cord reinforcement layer in the case where the end of the cord reinforcement layer is made close to the outer surface of the tire. This is because the buttress portion relatively often touches water when the tire is driven on a wet road surface. Accordingly, the moisture accelerates the deterioration of the belt cord, and this causes decrease in durability of the pneumatic tire. In addition, the buttress portion is deformed to a large extent when the tire is driven. For this reason, the cord reinforcement layer tends to be damaged from the end thereof when the end is made closer to the outer surface of the tire near the buttress portion. This also causes the decrease in durability of the pneumatic tire. To sum up, it is not possible to sufficiently obtain effects of improving durability of a pneumatic tire even if oxidation degradation of belt layers is prevented based on the cord reinforcement layer.